Process for producing tailor-made welded blanks

Abstract

A method for producing tailor-made welded blanks, which includes welding with a laser using a welding wire of one or more pairs of different coated steel plates having different thicknesses or strengths, wherein the welding wire contains 0.6 to 0.9 wt% C, 0.3 to 0.9 wt% Mn, 1.6 to 3.0 wt% Ni, Fe and other unavoidable impurities, and each of the coated steel plates has a base material 0.19 to 0.25 wt% C, 0.20 to 0.40 wt% Si, 1.10 to 1.60 wt% Mn, 0.03 wt% or less P, 0.015 wt% or less S, 0.10 to 0.60 wt% Cr, 0.0008 to 0.0050 wt% B, Fe and other unavoidable impurities.

Description

BACKGROUND OF THE INVENTIONTechnical field

[0001] Exemplary embodiments of the invention relate to a method for manufacturing customized welded blanks manufactured by joining steel plates having different thicknesses and materials, and more specifically to a method for manufacturing customized welded blanks capable of improving a quality of a welded portion and reducing a manufacturing time when manufacturing a customized welded blank using a coated steel plate. Description of the state of the art

[0002] In recent years, vehicle environmental and safety standards have been continuously tightened. To meet the demand for lightening, improving fuel efficiency and collision safety, high-strength steel, including advanced high-strength steel (AHSS), is being widely adopted in vehicles.

[0003] High-strength parts are used in vehicle body construction to cope with side impacts. In particular, the role of side pillars in battery protection during a side impact in an electric vehicle is very important, compared to the body of existing internal combustion engines. In this regard, super-high-strength steel is increasingly being used through HPF (hot press forming) technology.

[0004] However, collision absorption parts or elements can be broadly classified into two parts.

[0005] One is an energy absorption part that absorbs shocks that come from the outside by deforming it.

[0006] The energy absorption part is typically used on both the front of a front side member, the rear of a rear side member, and the bottom of a B-pillar.

[0007] The other is an anti-intrusion part or element that is hardly deformed. For example, since a passenger compartment occupied by passengers needs to be secured during a collision, the collision absorption part used in the passenger compartment is usually the anti-intrusion part.

[0008] The anti-intrusion part is typically used on both the rear of the front side member, the front of the rear side member and the top of the B-pillar.

[0009] The use of anti-intrusion parts employing HPF technology is rapidly increasing to improve collision safety. AHHS with a relatively high strain rate is used for the energy absorption part.

[0010] The combined shape of the energy absorption part and the anti-intrusion part is used in each of the front side member, the rear side member, and the B-pillar, and is manufactured by welding and forming the two parts.

[0011] In this case, a commonly used TWB (Tailor-Made Welded Blank) process involves a series of parts manufacturing processes in which steel plates of different thicknesses, strengths, and materials are cut into the required shape, like cutting a suit, the cut steel plates are welded, and the welded steel plates are then formed with a press. The TWB process is broadly composed of a cutting process for cutting steel plates, a welding process for laser welding the cut steel plates, and a stamping process for integrally processing the welded steel plates.

[0012] Because the TWB process welds various steel plates of varying thicknesses and materials, the welded element can have the properties required for each section. Additionally, the TWB process allows the various steel plates to be manufactured to precise dimensions, as well as a strong structure and a long service life, compared to a single steel plate. Therefore, the TWB process is used to manufacture structures such as vehicle body panels or electric rail vehicles.

[0013] In particular, the TWB method can achieve a reduction in the number of components, a vehicle body weight reduction, a manufacturing cost reduction, a quality improvement, a collision safety improvement, a vehicle body structure simplification and the like, compared with a conventional forming method in which parts are manufactured by cutting steel plates, forming each of the cut steel plates and then spot welding the formed steel plate, during the manufacture of structures such as vehicle body panels in automotive industrial fields that require a high production rate, low cost and low weight.

[0014] However, in a coated steel plate with an Al-Si or Zn coated layer, it is difficult to secure an austenite structure even when the coated steel plate is heated at a temperature of 900 to 950°C, and it is difficult to secure a martensite structure when the coated steel plate is quenched, respectively, because the coated layer is mixed into a welded portion during laser welding. Therefore, there is a problem that the material properties of the welded portion are deteriorated.

[0015] Accordingly, it is necessary to remove the applied layer from the coated steel plate in order to weld the coated steel plate. However, there is a problem in that the quality of the produced part deteriorates because rust is generated at the portion of the coated layer removed for welding. Furthermore, WO 2014 / 075824 A1 discloses a method for laser welding one or more workpieces made of hardened steel and using a welding wire. Other welding methods and welding wires are known from WO 2013 / 147035 A1 and US 2014 / 0 154 521 A1. SUMMARY OF THE INVENTION

[0016] An embodiment of the present invention relates to a method for manufacturing tailor welded blanks, which is suitable for increasing a quality of a welded portion even if a coated layer is not removed when a coated steel plate is welded by tailor welded blank welding.

[0017] Another embodiment of the present invention relates to a method of manufacturing tailor-made welded blanks in which a welded portion has a complete martensite structure after hot or warm stamping operations.

[0018] Other objects and advantages of the present invention will be understood from the following description and will become apparent upon reference to the embodiments of the present invention. It will also be readily understood by those skilled in the art to which this invention pertains that the objects and advantages of the present invention may be realized by the claimed instrumentalities and combinations thereof.

[0019] In accordance with one embodiment of the present invention, a method for manufacturing custom welded blanks includes welding one or more pairs of different coated steel plates, having different thicknesses or gauges, with a laser using a filler wire.

[0020] The welding wire may include a component by which a welded portion is transformed to have an austenite structure at a temperature of 900 to 950°C even though a coated layer of each of the coated steel plates is mixed into the welded portion.

[0021] The welding wire may contain C and Mn as austenite stabilizing elements.

[0022] The welding wire may further comprise Ni as an austenite stabilizing element.

[0023] The welding wire may contain 0.6 to 0.9 wt% C, 0.3 to 0.9 wt% Mn, 1.6 to 3.0 wt% Ni, and the balance Fe, and other unavoidable impurities.

[0024] Each of the coated steel plates may have an Al-Si coated layer and a base material may have 0.19 to 0.25 wt% C, 0.20 to 0.40 wt% Si, 1.10 to 1.60 wt% Mn, 0.03 wt% or less P, 0.015 wt% or less S, 0.10 to 0.60 wt% Cr, 0.0008 to 0.0050 wt% B, the balance Fe and other unavoidable impurities.

[0025] The method may further include forming a laser welded custom blank by hot stamping using the welding wire and then quenching it at a quenching rate of 40°C / s or more so as to convert a weld portion structure into a martensite structure. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] The features and advantages of the present invention will be more clearly understood from the following detailed description when considered in conjunction with the accompanying drawings in which: Fig. 1 is a graph showing a transformation curve according to an aluminum content and temperature in the prior art; Fig. 2 is a graph showing a movement of a transformation curve according to an increase in a content of an austenite stabilizing element; Fig. 3 is a graph showing a conversion curve according to an increase in a nickel content; and Fig. 4 is a graph showing a voltage test result according to a nickel content of a welding wire. DESCRIPTION OF SPECIFIC EMBODIMENTS

[0027] Exemplary embodiments of the present invention will be described in more detail below with reference to the accompanying drawings. However, the present invention may be embodied in different forms and should not be construed as limited to the embodiments set forth herein. Rather, the embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the present invention to those skilled in the art. Throughout the disclosure, the same reference numerals refer to the same elements throughout the different figures and embodiments of the present invention.

[0028] A main idea of ​​a method of manufacturing customized welded blanks according to an embodiment of the present invention is to prevent problems such as strength deterioration due to a component of a coated layer being mixed into a weld portion when manufacturing a customized welded blank having locally different material properties by joining one or more pairs of different coated steel plates having different materials or thicknesses by laser welding.

[0029] When coated steel plates are joined by laser welding in a manner where the coated layer of the welded portion is not removed, problems such as strength deterioration of the welded portion generally occur when the coated layer is melted and mixed into the welded portion. This is because the welded portion exhibits material properties different from those of the base material, as the welded portion contains a mixed component of the base material of the coated steel plate and the coated layer. Therefore, strength deterioration of the welded portion is caused.

[0030] Mainly, a boron steel plate with hardenability is used as a base material of the customized welded blank for hot stamping dies, and an Al-Si coated layer is mainly used as the coated layer.

[0031] Fig. Figure 1 is a graph showing a conventional transformation curve.

[0032] As in Fig. As shown in Figure 1, when the coated layer is mixed into the weld portion and an Al content of the weld portion is increased, it is impossible to secure a complete austenite structure even if the weld portion is heated at a temperature of 900 to 950°C, and the weld portion has a structure in which ferrite and martensite coexist with each other, instead of a complete martensite structure, even if the weld portion is quenched after hot stamping. Therefore, the strength deterioration of the weld portion is caused.

[0033] In the tailor-made welded blank used in the present invention, a base material containing 0.19 to 0.25 wt% of C, 0.20 to 0.40 wt% of Si, 1.10 to 1.60 wt% of Mn, 0.03 wt% or less of P, 0.015 wt% or less of S, 0.10 to 0.60 wt% of Cr, 0.0008 to 0.0050 wt% of B, the balance being Fe, and other unavoidable impurities such as Cu, Mo, Ti, Nb and the like other than the above-mentioned components, and an Al-Si coated layer is used.

[0034] In the method of manufacturing customized welded blanks according to the embodiment of the present invention, it is preferable that the weld portion has a complete austenite structure at a temperature of 900 to 950°C by adjusting components of the weld portion using a welding wire during laser welding.

[0035] Accordingly, the weld portion can have a desired material property by increasing its strength in such a way that the weld portion is transformed to have a full martensite structure by quenching the weld portion after the heat stamping.

[0036] Fig. 2 is a graph showing a movement of a transformation curve according to an increase in an austenite stabilizing element content.

[0037] As in Fig. 2, C and Mn are austenite stabilizing elements. The transformation curve is moved to the right side because a eutectoid temperature A C3 is decreased when the contents of the elements are increased and therefore an austenite region is increased.

[0038] Accordingly, the welding wire according to the embodiment of the present invention preferably has C and Mn as the austenite stabilizing elements.

[0039] Therefore, the weld portion has the full austenite structure when the weld portion is heated at the temperature of 900 to 950°C during heat stamping after welding, and the weld portion is transformed to have the full martensite structure when the weld portion is quenched after heat stamping.

[0040] It is preferable that the welding wire according to the embodiment of the present invention further comprises Ni as the austenite stabilizing element.

[0041] Fig. Figure 3 is a graph showing a transformation curve according to an increase in nickel content.

[0042] As in Fig. 3, a eutectoid temperature A C3 decreases when the nickel content of the welding wire is increased.

[0043] The welding wire according to the embodiment of the present invention preferably comprises 0.6 to 0.9 wt% of C, 0.3 to 0.9 wt% of Mn, 1.6 to 3.0 wt% of Ni, and the balance of Fe, and other unavoidable impurities such as Cu, Mo, Ti, Nb, and the like, other than the above-mentioned components.

[0044] This is because a ferrite structure coexists in the weld portion at the temperature of 900 to 950°C during welding, since an increase rate of an austenite region is low when the C content is lower than 0.6 wt%, and a rupture is generated at the weld portion during impact generation such as collisions, while a hardness and strength of the weld portion are excessively increased when the C content exceeds 0.9 wt%.

[0045] In addition, when Mn and Ni are outside the above composition range, the weld portion does not have the complete austenite structure at the temperature of 900 to 950°C. For this reason, there is a problem that the material properties of the weld portion of the finished product are changed after heat stamping, and defects such as cracking or fracture of the weld portion are generated.

[0046] Fig. Figure 4 is a graph showing a voltage test result according to the nickel content of the welding wire.

[0047] As in Fig. As shown in Figure 4, a base material fracture is generated during a stress test due to an increase in the weld portion when the Ni content of the welding wire is 1.6 to 3.0 wt%, whereas a weld portion fracture is generated during the stress test when the Ni content is less than 1.6 wt% or exceeds 3.0 wt%.

[0048] As in Fig. As shown in FIGS. 2 to 4, when the welding wire according to the embodiment of the present invention satisfies the above composition range, the weld portion has the complete austenite structure at the temperature of 900 to 950°C. Accordingly, it is possible to increase the strength of the weld portion by transforming the weld portion to have the complete austenite structure in such a manner that the weld portion is quenched after the heat stamping.

[0049] That is, the welding wire of the present invention can enable the weld portion after heat stamping to have the complete martensite structure by virtue of the austenite stabilizing elements contained in the welding wire, even if Al of the coated layer is mixed into the weld portion during welding. Since the hardness and strength of the weld portion are prevented from deteriorating even though welding is performed without removing the coated layer, the occurrence of weld portion fracture can be prevented.

[0050] In addition, in the method of manufacturing customized welded blanks according to the embodiment of the present invention, it is preferable that the weld portion is transformed to have the martensite structure by quenching the weld portion after the heat stamping at a quenching rate of 40°C / s or more.

[0051] In the present invention, it is preferable to initiate quenching immediately after heat stamping (within 0.5 seconds) and to set an average quenching rate during quenching to 40°C / s or more.

[0052] When the quenching rate is set to 40°C / s or less, it is difficult to refine the structure of the weld portion due to grain growth. As a result, it is difficult to transform the structure of the weld portion into the full martensite structure after quenching.

[0053] According to the exemplary embodiment of the invention, it may be possible to increase productivity and reduce manufacturing costs by shortening production time because removal of a coated layer and recoating processes are not required when a customized welded board is manufactured by laser welding.

[0054] Additionally, since a weld section has a complete martensite structure, a quality of a weld section can be improved.

[0055] While the present invention has been described with reference to the specific embodiments, it will be apparent to those skilled in the art that various changes and modifications can be made without departing from the spirit and scope of the invention as defined in the following claims.

Claims

[1] A method for producing tailor-made welded blanks, which includes welding with a laser using a welding wire of one or more pairs of different coated steel plates having different thicknesses or strengths, wherein the welding wire contains 0.6 to 0.9 wt% C, 0.3 to 0.9 wt% Mn, 1.6 to 3.0 wt% Ni, Fe and other unavoidable impurities, and each of the coated steel plates has a base material 0.19 to 0.25 wt% C, 0.20 to 0.40 wt% Si, 1.10 to 1.60 wt% Mn, 0.03 wt% or less P, 0.015 wt% or less S, 0.10 to 0.60 wt% Cr, 0.0008 to 0.0050 wt% B, Fe and other unavoidable impurities. [2] The method according to claim 1, wherein the welding wire has a component by which a welding portion is transformed to have an austenite structure at a temperature of 900 to 950°C when a coated layer of each of the coated steel plates is mixed into the welding portion. [3] A method according to any one of the preceding claims, wherein: each of the coated steel plates has an Al-Si coated layer. [4] A method according to any preceding claim, further including forming a laser-welded custom welded blank by heat stamping using the welding wire and then quenching the same at a quenching rate of 40°C / s or more so as to transform a weld portion structure into a martensite structure.

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